Co to co2 combustion promoter

ABSTRACT

The invention is directed to a CO to CO 2  combustion promoter comprising microsphere sized porous silica and/or alumina comprising particles further comprising on or more Group VIII noble metals wherein the noble metal is distributed in the particle as an eggshell such that a higher content of noble metal is present in the outer region of the particle as compared to the content of noble metal in the center of the particle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional under 35 U.S.C. § 121 of co-pendingU.S. application Ser. No. 17/176,222 filed Feb. 16, 2021, which is acontinuation under 35 U.S.C. § 120 of U.S. application Ser. No.17/043,031 filed Sep. 29, 2020 and now abandoned, which is a 35 U.S.C. §371 National Phase Entry Application of International Application No.PCT/US2019/024742 filed Mar. 29, 2019, which designates the U.S. andclaims benefit under 35 U.S.C. § 119(a) of N.L. Provisional ApplicationNo. 2020819 filed Apr. 24, 2018 and under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 62/651,295 filed Apr. 2, 2018, the contentsof which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The invention is directed to a CO to CO₂ combustion promoter comprisingmicrosphere sized porous alumina particles comprising one or more GroupVIII noble metals.

BACKGROUND OF THE INVENTION

Such CO to CO₂ combustion promoters are well known and are used in afluid catalytic cracking (FCC) unit. In such a FCC process fluidizablecatalyst particles are contacted in a conversion zone at hightemperatures with hydrocarbons such to crack higher boiling hydrocarbonsto lower boiling hydrocarbons, typically boiling in the gasoline range.The catalyst may be a crystalline zeolitic aluminosilicate component,usually an ion-exchanged form of a synthetic crystalline Faujasite, anda porous inorganic oxide matrix. This type of catalyst must beregenerated to low carbon levels, typically 0.5% or less, to assure thatthe catalyst particles possess desired activity and selectivity beforethe particles are recycled to the conversion zone, also referred to ascracking zone. In most regenerators the combustible solids deposited onthe spent solid catalyst particles from the cracking zone are burned ina confined regeneration zone in the form of a fluidized bed which has arelatively high concentration of catalyst particles (dense phase). Aregion of lower solids concentration (dilute phase) is maintained abovethe dense phase.

High residual concentration of carbon monoxide in flue gases fromregenerators has been a problem. The oxidation of carbon monoxide ishighly exothermic and can result in so-called “carbon monoxideafterburning” which can take place in the dilute catalyst phase, in thecyclones or in the flue gas lines. Afterburning has caused significantdamage to plant equipment. CO to CO₂ combustion promoters are added tothe FCC catalyst inventory of the regenerator with the objective toachieve a complete combustion of carbon monoxide in the regeneratordense phase and thereby avoiding this afterburning. The combustionpromoters are separate particles from the cracking catalyst particles.The combustion promoters will be circulated with the rest of the FCCcatalyst inventory from the regenerator to the riser, to the stripperand back to the regenerator.

Such CO to CO₂ combustion promoters are commonly made from impregnationof Group VIII noble metal or metals, commonly platinum and/or palladium,onto a porous alumina and/or other substrate microspheres of averageparticle size 60 to 90 microns, with physical properties very similar tothe base FCCU catalysts. Such impregnation results in a uniformdistribution of the Group VIII noble metal throughout the internal andexternal surfaces of the porous microsphere.

EP1879982 describes a CO to CO₂ combustion promoter for use in FCCcontaining platinum or palladium predominately present in the core ofthe particle and a metal active for catalysing NOx decomposition in ashell around the platinum or palladium.

US2005/0042158 describes a CO to CO₂ combustion promoter for use in FCCcontaining cerium oxide and a noble metal, like platinum and/or rhodium.

Other background publications are U.S. Pat. No. 6,165,933, US2005042158,U.S. Pat. Nos. 4,812,431, 4,300,997.

A disadvantage of all noble metal based CO to CO₂ combustion promotersis that relatively large amounts of noble metal are required to achievethe desired CO combustion in the FCC process. It is an objective of thepresent invention to provide a CO to CO₂ combustion promoter whichrequires less noble metal to achieve the same level of CO combustion inthe FCC process.

SUMMARY OF THE INVENTION

This is achieved with the following CO to CO₂ combustion promoter. A COto CO₂ combustion promoter comprising microsphere sized porous silicaand/or alumina comprising particles further comprising one or more GroupVIII noble metals wherein the noble metal is distributed in the particleas an eggshell such that a higher content of noble metal is present inthe outer region of the particle as compared to the content of noblemetal in the center of the particle. In one aspect of any of theembodiments, described herein is a CO to CO₂ combustion promotercomprising a particle, e.g., spherical particle, comprising i) silicaand/or alumina and ii) one or more Group VIII noble metals, wherein theone or more noble metals is distributed in the particle in a gradient,with the concentration of each noble metal increasing toward the outersurface of the particle relative to the center of the particle. In someembodiments, the particles are less than 1 mm across the largestdimension, e.g., they are microspheres. The concentration of the noblemetal can be zero, or any detectable concentration at the lowest pointof the gradient. The concentration of the one or more noble metals canincrease along the gradient at a constant or inconstant rate. In someembodiments, the particle can comprise two regions—a first inner regionnot comprising the one or more noble metals or a very low concentrationof such metals and a second outer region, referred to herein as an“eggshell” comprising the one or more noble metals.

In one aspect of any of the embodiments, described herein is a CO to CO₂combustion promoter comprising a particle, e.g., spherical particle,comprising i) a first region comprising porous silica and/or alumina andii) a second region comprising the surface of the particle and(eggshell) comprising one or more Group VIII noble metals, wherein theconcentration of one or more noble metals is greater in the secondregion than in the inner region and/or than in the pores of the innerregion. In some embodiments, the particles are less than 1 mm across thelargest dimension, e.g., they are microspheres. The concentration of thenoble metal can be zero, or any detectable concentration in the firstregion or in the pores of the first region.

Applicants found that the CO to CO₂ combustion promoter was active ascombustion promoter while the combustion promoter itself containedalmost only the Group VIII noble metal in an eggshell of the particle.Thus the same activity could be achieved using essentially significantlyless noble metal per particle. Without wanting to be limited by thefollowing theory applicants believe that the presence of the noble metalin the outer eggshell of a microsphere particle is sufficient to achievean active combustion promoter. This is because the CO to CO₂ oxidationas catalyzed by a Group VIII noble metal is a very rapid, essentiallyinstantaneous, reaction at the FCC regenerator temperatures ofapproximately 704° C. (1,300° F.) which is known as a diffusion limitedor a mass-transfer limited reaction. Thus, the noble metal in the centerof the particle is not active for CO to CO₂ oxidation as the reactantgases (CO and O₂) react before reaching this noble metal.

A further advantage with the eggshell of noble metal is that less NOxformation is achieved. This may be explained as follows. NOx formingreactions are slower and thus less diffusion limited than the CO to CO₂reactions. Thus, the prior art combustion promoters having a uniformdistribution and higher loading of the noble metal will generate moreNOx as this additional noble metal will catalyse NOx forming reactions.

Further, it is known the noble metal will sinter in time making thecombustion promoter less active for CO to CO₂ oxidation. The sinteredparticle will, however, still remain in the catalyst inventory of a FCCunit and continue to catalyse the NOx forming reactions. It has beenreported that the sintered noble metal even promotes the NOx formingreactions better than the fresh-non-sintered noble metal. The sinteredmetal within the entire particle will promote the NOx forming reactionsbecause these reactions are less diffusion limited. The noble metal ofthe combustion promoters according to the invention will also sinter.But because such a particle will contain considerably less noble metalthe catalytic activity for the NOx forming reaction of such degradedcombustion promoters will be less compared to the degraded combustionpromoters of the prior art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a cross-section of a state of the art combustion promoter.

FIG. 2 shows a cross-section of a combustion promoter according to theinvention.

FIG. 3 shows a graph with time on the x-axis after injecting a promoterin a FCC regenerator and on the y-axis the temperature change of thedilute phase of the FCC regenerator.

FIG. 4 shows the relative promoter concentrations of the combustionpromoter according to this invention and state of the art promoter onthe left hand y-axis and the flue gas NOx concentration in ppm on theright hand y-axis. The x-axis were the dates during the period the FCCregenerator was monitored.

DETAILED DESCRIPTION OF THE INVENTION

In this description the following terms are used having the followingdefinition.

FCC process: process to crack heavy oil fractions to lighter oilfractions by contacting the heavy oil fraction in a riser reactor wherethe heavy oil cracks to lighter oil in the presence of hot FCC catalystinventory and deposits coke on the FCC catalyst inventory, by separatinglight oil fractions from the catalyst inventory by means of cyclones andby stripping and wherein coke is removed from the catalyst inventory bycombustion in a regenerator to obtain a flue gas and hot catalystinventory for reuse in the riser.

FCC Unit: Installation comprising of a riser, a stripper and aregenerator and means to circulate FCC catalyst inventory from the riserto the stripper and to the regenerator and back to the riser.

FCC catalyst inventory or catalyst inventory: the total of solidparticles comprising of FCC catalyst and additives which are circulatedfrom the regenerator to the riser, to the stripper and back to theregenerator.

FCC catalyst: a silica-alumina comprising particle comprising a zeolite.

Spent FCC catalyst: deactivated FCC catalyst obtained from a FCC processcontaining coke depositions.

Equilibrium FCC catalyst: mixture of active and deactivated FCC catalystrepresenting an average activity of a stable running FCC process.

Eggshell catalysts are known for larger catalyst particles. Applicantnow discovered that it is possible to also form such an eggshelldistribution on microsphere sized porous particles. The microspheresized particles suitably have an average (D50) size of between 60 and 90micron as measured by laser light scattering, also referred to as laserdiffraction, using for example a Malvern Mastersizer 3000.

The eggshell or outer shell suitably has a depth from the outer surfacetowards the interior of the particle of between 1 to 10 microns. In thiseggshell, which includes the outer surface of the particle, suitablymore than 60 wt %, suitably more than 80 wt % and even more suitablymore than 90 wt % of the noble metal is present of the total of noblemetal present in the particle.

The silica and/or alumina comprising particle of the CO to CO₂combustion promoter may be a particle based on a predominately onlysilica particle. Examples of suitable silica particles are spray driedsilica particles. Such a silica particle may be impregnated as describedhere below with the noble metal or metals to obtain the promoteraccording to the invention.

The silica and/or alumina comprising particle of the CO to CO₂combustion promoter may also be a particle comprising both alumina andsilica. Suitable examples of such particles are FCC catalyst particleswhich may comprise between 30 and 70 wt % alumina, between 35 and 70 wt% silica. Such particles will also comprise of a zeolite, suitablyFaujasite or Type Y zeolite, and/or ZSM-5, and binders such as silicasol, alumina sol, pseudo-boehmite alumina or a clay-based matrix.Suitably equilibrium FCC catalyst particles or spent FCC catalystparticles are used as obtained from a fluidized catalytic cracking (FCC)process. The equilibrium or spent FCC catalyst is obtained from a FCCprocess wherein fresh FCC catalyst has deactivated to some degree. Suchcatalyst particles may thus find a suitable second use as CO to CO₂combustion promoter. By impregnating the FCC catalyst particles asdescribed here below with the noble metal or metals the promoteraccording to the invention may be obtained.

The silica and/or alumina comprising particle of the CO to CO₂combustion promoter is suitably an alumina particle and more preferablya gamma or theta alumina particle. Such an alumina starting particle mayconsist of predominately only alumina, suitably resulting in a CO to CO₂combustion promoter wherein the support, thus excluding the noblemetals, has an alumina content of above 95 wt % and more preferablyabove 99 wt %. The starting gamma or theta alumina particles suitablyhave an average (D50) size of between 60 and 90 micron, have a surfacearea (BET) of between 50 and 300 m²/g and preferably between 50-150 m²/gand a pore volume of between 0.05 and 0.50 mL/g and preferably between0.10-0.40 mL/g. Examples of suitable starting alumina particles areobtainable from Sasol, such as Puralox and Catalox. Such startingalumina particles may be impregnated as described here below with thenoble metal or metals to obtain the promoter according to the invention.

The Group VIII noble metal is suitably platinum, palladium, iridium,ruthenium and/or rhodium. Platinum can be preferred because of itsavailability. Palladium and rhodium can be preferred because theypromote the NOx forming reactions less than platinum. A problem in priorart devices is their availability. Because the combustion promoteraccording to the inventions requires less noble metal for achieving thesame activity noble metals like palladium and rhodium may be practicallyapplied.

The average content of noble metal per grams of combustion promoter mayrange from 1 to 5000 ppm and preferably between 100 and 1500 ppm.Locally, in the eggshell, this concentration will of course be larger.The concentration of noble metal in the eggshell may suitably be in thesame range as the concentration of noble metal in the prior artcombustion promoters which have the noble metal evenly distributedwithin the particle. The optimal content of noble metal will depend ontheir catalytic activity for promoting the CO to CO₂ combustion, whereinit is known that platinum is very active and will thus require a lowercontent than for example palladium which is known to be less active.

The concentration of the noble metal on the support is typically verylow. To establish whether the metals are homogeneously distributed orpresent as an eggshell it is possible to use Low Electron IonScattering. This technique (Platinum Metals Rev., 2010, 54, (2), 81 87)can establish surface concentrations already at 0.01 at %, which is inthe applicable range. By combining this surface technique together witha bulk chemical analysis technique such as X-ray fluorescence (XRF) orinductively coupled plasma (ICP) it can be established whether thesurface is enriched in noble metal, and the metal is predominantlypresent as an eggshell. For example, if the concentration at the surfaceis 2 times as high as the bulk concentration using a technique that hasa penetration depth of 10 micron, the metal distribution ispredominantly at the surface and hence is considered an eggshell typedistribution.

The combustion promoter may comprise co-catalytical compounds as forexample known for prior art combustion promoters. For the example ceriumoxide which may be added to provide oxygen storage or copper oxide toreduce NOx forming reactions. Such co-catalyst compounds are preferablypresent in the eggshell comparable to the noble metal distribution inthe particle. Preferably cerium oxide is distributed in the particle asan eggshell such that a higher content of cerium oxide is present in theouter region of the particle as compared to the content of cerium oxidein the centre of the particle. Accordingly, in some embodiments,described herein is a CO to CO₂ combustion promoter comprising aparticle (e.g., spherical particle) comprising i) silica and/or aluminaand ii) one or more Group VIII noble metals and one or moreco-catalytical compounds, wherein the metals and compounds of ii) aredistributed in the particle in a gradient, with the concentration ofeach noble metal and co-catalytical compound increasing toward the outersurface of the particle relative to the center of the particle. In someembodiments, the particle can comprise two regions—a first inner regionnot comprising the one or more noble metals and a second outer region,referred to herein as an “eggshell”, comprising the one or more noblemetals and one or more co-catalytical compounds.

Typically, the sensitivity to attrition of the CO to CO₂ combustionpromoter particles is about the same or even better than the FCCcatalyst inventory to which the promoter particles are added. Applicantshave now found that it is advantageous to use CO to CO₂ combustionpromoter particles which have a higher sensitivity to attrition than thetypical FCC catalyst. The advantage is that in this way deactivated COto CO₂ combustion promoter particles comprising sintered noble metal ormetals will reduce in size, by for example wear or fracture, quickerthan in the prior art processes. The smaller sized CO to CO₂ combustionpromoter particles will subsequently be removed from the FCC process viathe flue gas leaving the FCC regenerator. These fines in the flue gasmay be suitably removed from the flue gas by means of advance externalparticulate emissions control devices such as Electro-StaticPrecipitators (ESP) or wet-gas scrubbers. In this way the content ofdeactivated CO to CO₂ combustion promoter containing sintered noblemetals in the FCC catalyst inventory will be lower than when promoterparticles are used having a lower sensitivity to attrition. This resultsin turn in less NOx forming because the content of particles withsintered noble metal or metals is lower. See also the explanationregarding NOx formation above. The sensitivity may be expressed in theso-called Attrition Index as measured according to ASTM D-5757. TheAttrition Index of the CO to CO₂ combustion promoter for a sievefraction of combustion promoter particles of between 40 and 105 micronis preferably between 5 and 25 and more preferably between 10 and 20.Preferably this Attrition Index of the CO to CO₂ combustion promoter ishigher than the Attrition Index of the FCC catalyst itself. FCC catalystparticles typically have a sensitivity to attrition which results inthat the average catalyst residence time of the FCC catalyst in a FCCunit is between 2 weeks and 2 months. The residence time is an averageresidence time. The residence time of individual particles variesgreatly when one realises that typically on a daily basis catalyst isadded and withdrawn and partially lost to attrition in a FCC Unit.

The higher sensitivity to attrition of the CO to CO₂ combustion promoterpreferably results in that an average residence time of less than 5days, more preferably less than 3 days and even more preferably lessthan one day in the FCC catalyst inventory. Even shorter residence timeswill reduce NOx emissions even further. For the lowest NOx emissions atreasonable expenses, a residence time of half a day, or a quarter of aday would be considered optimal. At these lower residence timessignificant amounts of partially active combustion promoter will be lostas fines resulting in that more fresh CO to CO₂ combustion promoter willbe required to be added to the catalyst inventory. The skilled personwill be able to find the optimal Attrition Index for the CO to CO₂combustion promoter depending for example on the desired residence timeand the influence of the FCC unit itself to the attrition of the FCCcatalyst inventory. This process of finding the optimal Attrition Indexmay for example be an empirical iteration, where the attritionresistance of the additive is lowered (attrition increased) until theNOx emissions are lowered, and further reduction in the attritionresistance requires an increase in the addition rate of the combustionpromoter. In a FCC process the aged CO to CO₂ combustion promoter willbe removed from the process as fines and together with the catalystpurge which removes a part of the catalyst inventory from the FCCprocess. When the CO to CO₂ combustion promoter particles having thelower residence times than the 2 weeks and 2 months for FCC catalystsare used as described above it is preferred that more than 70 wt % ofthe CO to CO₂ combustion promoter as added to the process is removed asfines together with the flue gas wherein any remainder may be removedfrom the catalyst inventory by means of a catalyst withdrawal of the FCCcatalyst inventory.

Applicant believe that being able to influence, i.e. lower, theresidence time of part of the FCC catalyst inventory by increasing thesensitivity to attrition is novel and may also be applied in aninventive manner for other FCC additives and even for FCC catalystitself. Catalyst and additives used in the FCC process have always beendesigned with physical properties such as density and attrition thatmaximize the retention of the catalyst or additive in the circulatingFCC catalyst inventory. Such retention characteristics have beendesigned for these catalyst and additives irrespective of the life orhalf-life of the intended catalytic function of the individual additivesor catalyst and without consideration for any detrimental effects suchaged additives and/or FCC catalysts may have when left in the FCCcatalyst inventory. By influencing the sensitivity to attrition of theadditives, such as CO combustion promoters, and/or FCC catalyst one hasa means to minimise these detrimental effects of the aged additivesand/or catalyst.

Next to the earlier described CO to CO₂ combustion promoter, it may bepreferable that other FCC additives may also have a higher sensitivityto attrition than the FCC catalyst. Propylene enhancing additives basedon medium pore zeolites, such as ZSM-5, is initially more selective topropylene production but becomes more butylene selective as it ages dueto dealumination. Such additives are well known and for exampledescribed in Magee and Mitchell (editors), Studies in Surface Scienceand Catalysis vol. 76, Elsevier Amsterdam 1993. Both the fresh ZSM-5based additive and the deactivated additive are in competition in theFCC catalyst inventory for the same feed molecules to crack. Therefore,the presence of (partially) deactivated ZSM-5 reduces the overallpropylene selectivity and enhances the butylene selectivity. For maximumpropylene selectivity it is advantageous to limit the residence time ofthe ZSM-5 based additive in the FCC catalyst inventory to below 7 days,more preferred below 5 days and even more preferred to below 3 days. Theresidence time may suitably be established using chemical markers. Forexample, when to catalysts are added each at 5 tons per day to an FCCunit, with an inventory of 100 tons, the average residence time ininventory would be when all physical properties are equal 100/(5+5)=10days. When one of the catalysts or additives is provided with a chemicalmarker, and only half of that marker is present in the catalyticinventory, that means the residence time is half of the averageresidence time of the total inventory, i.e. 5 days.

Y zeolite used in FCC catalysts and in some FCC additives may lose theirhydrogen transfer capability and other desirable properties as they age,going through various phases of reduced and/or undesirable catalyticfunctionality until they finally become catalytically inert. By choosinga suitable Attrition Index for such Y zeolite based FCC catalystparticles and/or Y zeolite based additives one may avoid such reducedand/or undesirable catalytic functionality of the aged particles. Thisis because their residence time is reduced.

The invention is also directed to a FCC process wherein one or more FCCcatalysts and/or FCC additives in the catalyst inventory have a shorterresidence time in the catalyst inventory than the residence time of amore catalytically stable FCC catalysts and/or FCC additives and whereinthe one or more FCC catalysts and/or FCC additives in the catalystinventory having a shorter residence time are more than 70 wt % removedfrom the process as fines in the flue gas shorter while the remainder isremoved from the FCC process via a catalyst withdrawal of part of theFCC catalyst inventory.

The invention is also directed to a FCC process wherein one or more FCCcatalysts and/or FCC additives in the catalyst inventory have a shorterresidence time in the catalyst inventory than the residence time of amore catalytically stable FCC catalysts and/or FCC additives and whereinthe shorter residence time is a result of a higher sensitivity toattrition of said FCC catalysts and/or FCC additives as compared to thesensitivity to attrition of the more catalytically stable FCC catalystsand/or FCC additives.

The invention is also directed to a FCC unit comprising a catalystinventory comprising:

-   -   a. a first FCC catalyst, and    -   b. a second FCC catalyst and/or additive having a catalytic        active component having a catalytic activity,    -   whereby the catalytic activity of the second FCC catalyst and/or        additive is negatively altered by the effects on the        catalytically active component, whereby the second FCC catalyst        and/or additive is more sensitive to attrition and has a shorter        residence time in the catalyst inventory as compared to the        first FCC catalyst.

In the above FCC unit the additive may be a CO to CO₂ combustionpromoter, more specially a CO to CO₂ combustion promoter wherein atleast one of the catalytically active components of the additivecomprises a metal and wherein the catalytic effect of the metal isnegatively altered by sintering of the metal. The metal or metals may beas described above for the promoter according to the invention. Themetal may be distributed homogenously or according to a gradient. Thepromoter is preferably a CO to CO₂ combustion promoter is a CO to CO₂combustion promoter according to this invention. The additive may alsobe ZSM-5 or a ZSM-5 comprising additive and wherein the catalytic effectof the ZSM-5 is negatively altered by dealumination of the zeolite.

The CO to CO₂ combustion promoter having a gradient in metal content maybe obtainable by known processes to prepare eggshell type of catalystsas for example described in WO2016/151454. The CO to CO₂ combustionpromoter is suitably obtainable by a process comprising the followingsteps (a) introducing a medium in the pores of starting porous silicaand/or alumina comprising particles to obtain filled porous particlesand (b) contacting the filled porous particles with an aqueous saltsolution of the Group VIII noble metal or metals wherein the Group VIIInoble metal deposits predominantly in in the outer region of theparticles thereby obtaining intermediate eggshell particles andoptionally (c) dry and/or calcine the intermediate eggshell particles.The invention is also directed to this process as such.

The starting porous silica and/or alumina comprising particles may bethe aforementioned starting porous particles described above.

By introducing a medium which does not contain a Group VIII noble metalor noble metal salt in step (a) the pores of the porous alumina particleand especially the pores in the centre of the porous silica and/oralumina comprising particle will be filed with this medium. When theobtained filled porous particle is contacted in step (b) with an aqueoussalt solution of the Group VIII noble metal or metals the pre-filledpores in the centre of the particle will be more difficult to reach.This results in that the noble metal is deposited predominately in theeggshell of the particle.

The medium in such a process may be water, an oil, a solvent for thenoble metal salt or a liquid in which the noble metal salt barelydissolves. It is understood that the medium preferably does not containthe noble metal or its salt to avoid that noble metal deposits in thecentre of the particle. The water may contain other additives whichsuitably do not deposit on the alumina surface of the pores. Suchadditives may be gel-forming additives which make it even more difficultfor the aqueous salt solution of the Group VIII noble metal or metals toenter the pores in the centre of the particle. Alternatively, the filledporous particles may be reduced in temperature such that the water inthe pores solidifies. When such a particle is contacted in step (b) withthe aqueous salt solution of the Group VIII noble metal or metals thedeposition of metals will take place in the eggshell where any frozenwater will first melt and not in the pores which contain the frozenwater which will melt at a later moment in time.

For performing step (a) it is important to know the volume of the poresof the starting porous silica and/or alumina comprising particles, alsoreferred to as the support, which are to be filled with medium. The porevolume may be measured using nitrogen and/or quick porosimetry tomeasure micro-, meso- and macro-pore volumes. A preferred method is toestablish the “water pore volume”. In this the particles are fullysoaked in excess water. With the aid of a laboratory centrifuge theexcess water that is not contained in the pores is removed. Thedifference in weight of the water equals the pore volume as occupied bythe water. When no centrifuge is available, the pore volume can beestablished by adding water drop wise to a dry powder. The powder iswell mixed during the addition of water. The moment when the powder nolonger adsorbs the water and mixing of the place where the drop impactedwith dry parts of the powder, the powder is considered to be fullysaturated. This method is comparable to the incipient wetness techniquebut is now used to establish the pore volume of the support.

In step (a) the support is pre-contacted with a liquid medium,preferably water. The liquid can be a solvent for the salt, such aswater, or a non-solvent, such as paraffinic oil. The quantity of theadded medium is suitably between 50 and 100 vol % of the pore volume ofthe support, as established in the steps mentioned above. More preferredthe quantity of the added medium is at least 60% and even more preferredis at least 70% of the pore volume of the support. Higher than 80%filling of the pores will be possible, but this does require skill andcareful execution to homogeneously distribute the liquid. Pre-fillingthe pores to 90%, or even up to 95% will help to achieve the highestactivity for the noble metal but will require a careful execution of thepre-contacting step. A 100 vol % filing is preferred when one desires toonly deposit noble metals on the outer surface of the particle. The keyhere is the slow addition of the liquid medium to the support whilemixing and/or agitating the support. An alternative can be condensationof the pre-contacting liquid in the pores, using the appropriate partialpressures to fill a certain fraction of the pores. The pre-contactingliquid can potentially be solidified via the addition of a gellingagent, by lowering the temperature, potentially even freezing thepre-contacting liquid in the pores, or a combination of both. Anothermethod of adding the medium is to saturate the particles in a fluid bedin a stream containing the medium, where the medium can condense in thepores. The medium will be homogeneously distributed in this way over allthe particles.

The higher the fraction of pores filled in the initial liquid medium themore the noble metal will be concentrated on an outer eggshell resultingin a more active combustion promoter. The noble metal salt will be addedin the remainder of the pore volume in step (b). For example, when 80vol % of the pores are filled with the liquid medium, at most 20 vol %of pores can be filled with the noble metal solution. When the total ofthe 20 vol % of the remaining pore volume is used to add the noble metalthis method resembles a two-step incipient wetness technique. If lessthan all the pore volume is used to add the noble metal, the methodresembles a two-step pore volume impregnation.

Contacting in step (b) may be performed by state of the art methodsknown for depositing metals on the surface of silica and/or aluminacomprising particles and the surface of the pores of the silica and/oralumina comprising particles. In this method the salt solution of theGroup VIII noble metal is added in about the same volume as theremaining pores to be filled with the salt solution. In step (b) of thepresent process the filled porous particles are contacted with a volumeof the aqueous salt solution of the Group VIII noble metal or metalswhich is less than the total volume of the pores of the porous silicaand/or alumina comprising particles. This because the pore volume in thecentre of the particles is already filled with the medium. In thismanner only the pores in the outer shell of the particle, i.e. theeggshell, are filled with the aqueous salt solution and the noble metalpredominately deposit in the eggshell of the particle. The contact timein step (b) is preferably at most 30 minutes, more preferred at most 20minutes, even more preferred at most 10 minutes. Contact times of atmost 5 minutes or even at most 3 minutes may be even more preferred. Forexample, a one minute contact time could be suitable when the pores arefilled for 100 vol %. The impregnated particles and the aqueous saltsolution may be separated by means of filtration, for example using afilter, covered with a filter cloth to contain the small supportparticles. The filtration may be performed by means of a filter press, abelt filter or any other filtration device suitable to separate thequantity of catalyst prepared from the liquid slurry. The filtrate maycontain noble metal salts which are suitably recovered and reused tocontact a next batch of filled porous particles.

In step (c) the particle containing the deposited noble metal asobtained in step (b) may be optionally dried and/or calcined. Dryingsuitably takes place at temperatures between 100 to 250° C. (212 to 482°F.) and calcination suitably takes place at a temperature of between 250and 750° C. (482 to 1382° F.). Drying and/or calcination may be with orwithout vacuum. Drying is preferred to limit sintering of the noblemetal or metals.

Prior to or after performing steps (a)-(b) the starting silica and/oralumina comprising particles or impregnated particles can also besubjected to a process comprising steps (a)-(c) wherein in step (b) thefilled porous particles are contacted with an aqueous salt solution of acerium salt instead of the aqueous salt solution of the Group VIII noblemetal or metals Group VIII noble metal to obtain particles in step (c)comprising cerium oxide which is distributed in the particle as aneggshell such that a higher content of cerium oxide is present in theouter region of the particle as compared to the content of cerium oxidein the centre of the particle.

The invention is also directed to the use of the CO to CO₂ combustionpromoter as described above in a fluid catalytic cracking (FCC) unit.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 shows a state of the art combustion promoter where the Group VIIImetal is distributed homogeneously throughout an alumina microspheres ofaverage particle size 60 to 90 microns (twice the radius r). The aluminasupport has mesopores (pores less than 50 nanometers) distributedthroughout the particle. The capillary effects, both hydraulic andevaporative, in mesopores are well documented and are characterized byextremely rapid uptake of any liquid, including one with dissolved noblemetal salts. This rapid uptake into the pores of the support is whatresults in the homogenous distribution of the noble metals with theimpregnation methods previously employed and documented to manufactureFCC CO combustion promoters.

FIG. 2 shows a combustion promoter as prepared according to process ofthe invention. The alumina microsphere has the same dimension andmesopore structure as the alumina microsphere of FIG. 1. The core of theparticle contains little to no Group VIII metals and an outer eggshellwhich may have a thickness (d) of between 1 to 10 microns will containalmost all of the Group VIII metal.

The invention will be illustrated by means of the following non-limitingexamples.

Example 1

A two-step impregnation was carried out using a high purity gammaalumina support as purchased from Sasol. In the first step, 10 g ofalumina powder was impregnated with water to fill 90% of the porevolume. At a pore volume of 0.5 mL/g, 4.5 mL of water was required for10 g of alumina powder. For the second step, the wet alumina particlesfrom step 1 with 90% of the pore volume filled with water wereimpregnated with a solution of palladium nitrate and dried to achieve aneggshell of palladium on the particle surface. The second impregnationstep was completed before the pre-filled alumina particles were dried.After completing both impregnation steps, the impregnated particles werecalcined at 600° C. for 1 hour to form a particle with the compositionin accordance with this invention.

Example 2

0.1 g of the composition of Example 1 was mixed with 10 g of acommercial FCC catalyst. The mixture was then used to measure theoxidation of CO in a fluid bed reactor fitted with a thermocouple. Thecatalyst bed was heated to 600° C. prior to the start of CO/O₂ flow.Oxidation response was measured using a mixture of 1.8 v/v % CO and 0.9v/v % O₂, balanced with He, under a gas flow rate of 1000 cc/min. Theoxidation response is a measure of the amount of CO which is combusted(oxidized) to CO₂.

Comparative Experiment A

Example 2 was repeated using a state of the art FCC combustion promotercontaining twice the amount of palladium as compared to the promoter ofExample 1. In the state of the art FCC combustion promoter palladium washomogenously distributed over the particle. The CO oxidation responsefor the composition of Example 1 was comparable to the state of the artFCC combustion promoter despite having half the palladium loading.

Comparative Experiment B

Example 2 was repeated using a lab prepared FCC combustion promoter withthe same palladium loading as in Example 1 and prepared by standardhomogeneous impregnation method. This resulted in a homogeneousdistribution of palladium throughout the alumina support. The COoxidation response for the composition of Example 1 was better than thelab prepared FCC combustion promoter.

Example 3

A two-step impregnation was carried out using a high purity gammaalumina support as purchased from Sasol. In the first step, 10 g ofalumina powder was impregnated with water to fill 90% of the porevolume. At a pore volume of 0.5 mL/g, 4.5 mL of water was required for10 g of alumina powder. For the second step, the wet alumina particlesfrom step 1 with 90% of the pore volume filled with water wereimpregnated with a solution of platinum nitrate and dried to achieve aneggshell of platinum on the particle surface. The second impregnationstep was completed before the pre-filled alumina particles were dried.After completing both impregnation steps, the impregnated particles werecalcined at 600° C.

Example 4

Example 2 was repeated using 0.1 g of the composition of Example 3.

Comparative Experiment C

Example 4 was repeated using a state of the art FCC combustion promotercontaining twice the amount of platinum as compared to the promoter ofExample 3. In the state of the art FCC combustion promoter, platinum washomogenously distributed throughout the particle. The CO oxidationresponse for the composition of Example 3 was comparable to thecommercial FCC combustion promoter despite having half the platinumloading.

Comparative Experiment D

Example 4 was repeated using a lab prepared FCC combustion promoter withthe same platinum loading as in Example 3 and prepared by standardhomogeneous impregnation method. This resulted in a homogeneousdistribution of platinum throughout the particle. The CO oxidationresponse for the composition of Example 3 was better than the labprepared FCC combustion promoter.

Example 5

Three alumina supports with varying attrition indices were provided bycatalyst manufacturers. The attrition index of the three aluminasupports ranged from 2 to 25 as measured according to ASTM D-5757. Atwo-step impregnation was carried out for each of the three as suppliedalumina powders. In the first step, 10 g of alumina powder wasimpregnated with water to fill 90% of the pore volume. At a pore volumeof 0.5 mL/g, 4.5 mL of water was required for 10 g of alumina powder.For the second step, wet alumina particles with 90% of the pore volumefilled with water were impregnated with a solution of palladium nitrateand dried to achieve an eggshell of palladium on the particle surface.The second impregnation step was completed before the pre-filled aluminaparticles were dried. After completing both impregnation steps, theimpregnated particles were calcined at 600° C. for 1 hour to form aparticle with the composition in accordance with this invention.

Example 6

0.1 g of each of the compositions of Example 5 was mixed with 10 g of acommercial FCC catalyst to prepare 3 different mixtures. Each mixturewas then used to measure the oxidation of CO in a fluid bed reactorfitted with a thermocouple. The catalyst bed was heated to 600° C. priorto the start of CO/O₂ flow. Oxidation response was measured using amixture of 1.8 v/v % CO and 0.9 v/v % O₂, balanced with He, under a gasflow rate of 1000 cc/min.

Comparative Experiment E

For comparison Example 6 was repeated but now with a state of the artFCC combustion promoter and lab synthesized FCC combustion promoterprepared by standard homogeneous impregnation method. The CO oxidationresponse for each of the compositions of Example 5 as measured inExample 6 was comparable to the commercial FCC combustion promoterdespite having half the palladium loading. Further, the CO oxidationresponse for each of the compositions of Example 5 as measured inExample 6 was enhanced compared to lab prepared FCC combustion promoterwith the same palladium loading prepared by standard homogeneousimpregnation method. Further, the reduction in attrition resistance ofthe alumina support did not reduce the CO oxidation response.

Example 7

136 kg (300 lbs) of palladium-based CO combustion promoter according tothe invention (Promoter A) was supplied to Refinery A. During a one weektrial, Refinery A evaluated the performance in comparison to a state ofthe art CO combustion promoter (Promoter B) that has approximately twicethe palladium content with the palladium distributed homogenouslythroughout the particles. Afterburn control was maintained using 20%less Promoter A (line (a)) compared to Promoter B (line (b)) asdemonstrated in FIG. 3.

Example 8

136 kg (300 lbs) of palladium-based CO combustion promoter according tothe invention (Promoter C) was supplied to Refinery B. During a two weektrial, Refinery B evaluated the performance in comparison to a state ofthe art CO combustion promoter (Promoter D) that has approximately twicethe palladium content with the palladium distributed homogenouslythroughout the particles.

Refinery B provided comparative data from the trial, which included datafrom three weeks prior to the start of the Promoter C trial whenPromoter D was used for comparison. The first week of Promoter C trialis the transition from Promoter D, and therefore the second week ofPromoter C data was used for comparison purposes and summarized in Table1.

TABLE 1 Promoter D Promoter C 3 week 2nd week average average Flue GasO₂ mol % 1.2 1.4 Flue Gas CO ppm 213.0 140.7 Flue Gas NOx ppm 52.2 36.2CO Promoter Additions kg/day 11 9

Average flue gas composition and CO Promoter addition rates are given inTable 1. Promoter C controlled CO to lower levels compared to Promoter Ddespite having half the palladium content. Promoter C and Promoter Dboth controlled afterburn to acceptable and safe limits. Promoter Cachieved this with half the palladium loading and an 18% lower additionrate. NOx emissions declined as Promoter C replaced the Promoter D inthe FCC unit inventory, declining by over 30%. This result is attributedto less total palladium, both fresh and sintered, present in thecatalyst inventory.

The NOx emissions are shown in FIG. 4 where the relative promoterconcentrations of Promoter C and Promoter D are on the left hand y-axisand the flue gas NOx concentration in ppm (e) on the right hand y-axis.The x-axis were the dates the regenerator was monitored. FIG. 4 showsthe NOx reduction over time when the relative content of Promoter Ddecreases as Promoter C increases. The content is expressed as apercentage of the maximum concentration of Promoter D. On January 20-21,Promoter D was accidently injected and an increase in NOx emissions isobserved.

Refinery B conducted comparative shot tests of Promoter C and Promoter Dby injecting 13.6 kg (30 lb) of each promoter. Each injection of 13.6 kg(30 lb) of promoter reduced the FCC afterburn by 5.6° C. (10° F.). NOxemissions from the injections of Promoter C and Promoter D weredifferent. 13.6 kg (30 lb) of Promoter D increased NOx emissions from 45to 67 ppm, and 180 minutes after injection the NOx emissions remained at55 ppm. 13.6 kg (30 lb) of Promoter C increased NOx emissions from 45 to54 ppm, and 180 minutes after injection the NOx emissions remained at 50ppm. Therefore, Promoter C has a reduced impact on NOx emissions.

What is claimed herein is:
 1. A process to prepare a CO to CO₂combustion promoter comprising eggshell microsphere sized porousparticles, the process comprising: (a) introducing a medium in the poresof a plurality of starting microsphere sized porous particles, whereineach starting microsphere sized porous particle has a diameter of lessthan 1 mm and independently comprises silica, alumina, or mixturesthereof; (b) contacting the microsphere sized porous particles with anaqueous salt solution of one or more Group VIII noble metals, wherebythe one or more Group VIII noble metals deposit such that a higherconcentration of the one or more Group VIII noble metals is present inthe outer region of the microsphere sized porous particle as compared tothe concentration of the one or more Group VIII noble metals in thecentre of the microsphere sized porous particle, thereby obtainingeggshell microsphere sized porous particles.
 2. The process of claim 1,further comprising (c) drying and/or calcining the eggshell microspheresized porous particles.
 3. The process of claim 1, further comprising(c) drying the eggshell microsphere sized porous particles.
 4. Theprocess of claim 1, further comprising (c) vacuum drying the eggshellmicrosphere sized porous particles.
 5. The process of claim 1, whereinthe starting microsphere sized porous particles are gamma aluminaparticles having an average (D50) size of between 60 and 90 μm, have asurface area (BET) of between 50-300 m²/g, and a pore volume of between0.05 and 0.50 mL/g.
 6. The process of claim 1, wherein the medium instep (a) is water not containing a Group VIII noble metal salt.
 7. Theprocess of claim 1, wherein in step (b) the microsphere sized porousparticles are contacted with a volume of the aqueous salt solution ofthe one or more Group VIII noble metals which is less than the totalvolume of the pores of the starting microsphere sized porous particles.8. The process of claim 1, wherein the one or more Group VIII noblemetals comprises one or more of platinum, palladium, iridium, rutheniumand rhodium.
 9. The process of claim 8, wherein the one or more GroupVIII noble metals comprises platinum and palladium.
 10. The process ofclaim 8, wherein the one or more Group VIII noble metals comprisesplatinum.
 11. The process of claim 8, wherein the one or more Group VIIInoble metals comprises palladium.
 12. The process of claim 1, whereinthe starting microsphere sized porous particles comprise silica.
 13. Theprocess of claim 1, wherein the starting microsphere sized porousparticles comprise alumina.
 14. The process of claim 1, wherein theplurality of starting microsphere sized porous particles comprises afirst group of starting microsphere sized porous particles comprisingsilica and a second group of starting microsphere sized porous particlescomprising alumina.
 15. The process of claim 1, wherein the startingmicrosphere sized porous particles comprise silica and alumina.
 16. Theprocess of claim 1, wherein the starting microsphere sized porousparticles collectively have an average (D50) size of between 60 and 90microns as measured by laser diffraction.
 17. The process of claim 1,wherein the starting microsphere sized porous particles are gamma ortheta alumina particles.
 18. The process of claim 1, wherein thestarting microsphere sized porous particles are spray dried silicaparticles.
 19. The process of claim 1, wherein the an aqueous saltsolution of one or more Group VIII noble metals further comprises atleast one of cerium oxide and copper oxide, whereby the at least one ofcerium oxide and copper oxide deposit such that a higher concentrationof the at least one of cerium oxide and copper oxide is present in theouter region of the microsphere sized porous particle as compared to theconcentration of the at least one of cerium oxide and copper oxide inthe centre of the microsphere sized porous particle.
 20. The process ofclaim 1, wherein the pores of the plurality of starting microspheresized porous particles have a pore volume, and the medium introduced inthe pores in step (a) has a volume of between 50 and 100% volume of thepore volume.
 21. A FCC unit comprising a catalyst inventory comprising:a. a first FCC catalyst, and b. a second FCC catalyst and/or additivehaving a catalytic active component having a catalytic activity, wherebythe catalytic activity of the second FCC catalyst and/or additive isnegatively altered by the effects on the catalytically active component,whereby the second FCC catalyst and/or additive is more sensitive toattrition and has a shorter residence time in the catalyst inventory ascompared to the first FCC catalyst.
 22. The FCC unit of claim 21,wherein the additive is a CO to CO₂ combustion promoter comprisingmicrosphere sized porous particles, each microsphere sized porousparticle having a diameter of less than 1 mm, and independentlycomprising: silica, alumina, or mixtures thereof; and one or more GroupVIII noble metals distributed in the particle as an eggshell such that ahigher concentration of the one or more Group VIII noble metals ispresent in the outer region of the microsphere sized porous particle ascompared to the concentration of the one or more Group VIII noble metalsin the centre of the microsphere sized porous particle.
 23. A fluidcatalytic cracking (FCC) unit catalyst inventory comprising a CO to CO₂combustion promoter comprising microsphere sized porous particles, eachmicrosphere sized porous particle having a diameter of less than 1 mm,and independently comprising: silica, alumina, or mixtures thereof; andone or more Group VIII noble metals distributed in the particle as aneggshell such that a higher concentration of the one or more Group VIIInoble metals is present in the outer region of the microsphere sizedporous particle as compared to the concentration of the one or moreGroup VIII noble metals in the centre of the microsphere sized porousparticle.
 24. A FCC process wherein one or more FCC catalysts and/or FCCadditives in the catalyst inventory have a shorter residence time in thecatalyst inventory than the residence time of a more catalyticallystable FCC catalysts and/or FCC additives and wherein the shorterresidence time is a result of a higher sensitivity to attrition of saidFCC catalysts and/or FCC additives as compared to the sensitivity toattrition of the more catalytically stable FCC catalysts and/or FCCadditives.
 25. A CO to CO₂ combustion promoter comprising porousparticles, each porous particle having a diameter of less than 1 mm, andindependently comprising: silica, alumina, or mixtures thereof; and oneor more Group VIII noble metals distributed in the particle such that ahigher concentration of the one or more Group VIII noble metals ispresent in the outer region of the porous particle as compared to theconcentration of the one or more Group VIII noble metals in the centreof the porous particle.